The present invention relates to an illumination optical apparatus, an exposure apparatus, and a device manufacturing method, and more particularly, to an illumination optical apparatus suitable for use as an exposure apparatus that manufactures devices such as semiconductor device and a liquid crystal display device in a lithography process.
In this type of typical exposure apparatus, light emitted from a light source passes through a fly's eye lens (or micro-lens array), which serves as an optical integrator, and forms a secondary light source, which serves as a substantial planar light source formed by many light sources. Light from the secondary light source, which is converged by a condenser lens, illuminates in a superimposed manner a mask including a predetermined pattern.
The light transmitted through the pattern of the mask passes through a projection optical system and forms an image on a wafer. In this manner, the mask pattern is projected and exposed (transferred) onto the wafer. The pattern formed on the mask is highly integrated. Thus, the illumination distribution on the wafer must be uniform to accurately transfer the fine pattern onto the wafer.
In the prior art, a technique in which a circular secondary light source is formed at a rear focal plane of a fly's eye lens, and the size of the secondary light source is varied to change the illumination coherency σ (σ value=emission side numerical aperture of illumination optical system/incidence side numerical aperture of projection optical system) has been attracting attention. Further, a modified illumination technique of forming a annular-shaped or quadrupole secondary light source on the rear focal plane of the fly's eye lens to enhance the depth of focus and resolving power of a projection optical system described in Japanese Laid-Open Patent Publication No. 2002-231619 and corresponding U.S. Patent Publication No. 2004/0263817A1.
In the modified illumination technique, for example, for annular illumination, a conical axicon system including two prism members is used to change the outer shape (annular ratio etc.) of the annular illumination field (annular light intensity distribution), which is formed at an incidence plane (illumination pupil plane) of the fly's eye lens. This changes the outer shape of the annular-shaped secondary light source formed at or near the rear focal plane of the fly's eye lens. In recent exposure apparatuses, it is desired that exposure be performed in a high σ state in which the σ value is approximated to 1 in, for example, annular illumination.
However, if the two prism members in the conical axicon system is spaced apart from each other in annular illumination, the profile (cross-sectional shape) of the annular light intensity distribution on an illumination pupil plane (and ultimately, the pupil plane of the projection optical system) will be shaped with a distorted periphery. Such a shaped would differ from the ideal top-hat shape. In such a case, in the annular light intensity distribution at the pupil plane of the projection optical system, the light at the peripheral portion would not pass through an aperture of the pupil plane of the projection optical system. Such light would be shielded and become stray light thereby lowering the imaging capability of the projection optical system.